A polyethylene-based resin foam is generally used as a packing material owing to its high elasticity and excellent impact resistance. The polyethylene-based resin foam, however, has weakness that its stiffness and compressive strength are low. On the other hand, a styrene-based resin foam is excellent in stiffness, but has a weakness that it is brittle.
To overcome such weakness, Japanese Examined Patent Publication No. SHO 51(1976)-46138 (Patent Document 1), Japanese Examined Patent Publication No. SHO 52(1977)-10150 (Patent Document 2), Japanese Examined Patent Publication No. SHO 58(1983)-53003 (Patent Document 3) and Japanese Unexamined Patent Publication No. SHO 62(1987)-59642 (Patent Document 4) disclose methods in which polyethylene-based resin is impregnated with a styrene-based monomer and polymerization is performed to obtain styrene-modified polyethylene-based resin expandable beads.
As an expanded molded article that can be obtained by heat expanding small particles of styrene-modified polyethylene-based resin foam in a mold and fusing the particles together, Japanese Unexamined Patent Publication No. HEI 7(1995)-80873 (Patent Document 5) discloses an expanded molded article of styrene-modified polyolefin-based resin in which small particles are fused together so as to have 10% to 40% voids among themselves.
[Patent Document 1] Japanese Examined Patent Publication No. SHO 51(1976)-46138
[Patent Document 2] Japanese Examined Patent Publication No. SHO 52(1977)-10150
[Patent Document 3] Japanese Examined Patent Publication No. SHO 58(1983)-53003
[Patent Document 4] Japanese Unexamined Patent Publication No. SHO 62(1987)-59642
[Patent Document 5] Japanese Unexamined Patent Publication No. HEI 7(1995)-80873